Cycle here is more of a batch size. You can probably leave this as the default. It's how many emails get sent at a time. This is because some mail servers will scream at you if you try to send out 1,000 emails in a short period of time. Breaking it down into batches with a delay helps. So instead of sending 1,000 emails in the space of 2 minutes, for example, it will send 100 emails, wait 10 minutes, then send another 100, wait 10 minutes, etc.
Hope that clears things up.